Full color organic electroluminescent (EL), also known as organic light-emitting devices or (OLED), have been recently been demonstrated as a new type of flat panel display. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic EL medium sandwiched between these electrodes to support charge recombination that yields emission of light. An example of an organic EL device is described in commonly-assigned U.S. Pat. No. 4,356,429. In order to construct a pixilated display device such as is useful, for example, as a television, computer monitor, cell phone display or digital camera display, individual organic EL elements may be arranged as an array of pixels in a matrix pattern. To produce a multi-color display, the pixels are further arranged into subpixels, with each subpixel emitting a different color. This matrix of pixels can be electrically driven using either a simple passive matrix or an active matrix driving scheme. In a passive matrix, the organic EL layers are sandwiched between two sets of orthogonal electrodes arranged in rows and columns. An examples of a passive matrix driven organic EL devices is disclosed in commonly-assigned U.S. Pat. No. 5,276,380. In an active matrix configuration, each pixel is driven by multiple circuit elements such as transistors, capacitors, and signal lines. Examples of such active matrix organic EL devices are provided in U.S. Pat. No. 5,550,066 (commonly-assigned); U.S. Pat. Nos. 6,281,634 and 6,456,013.
Full color OLED devices are also known in the art. Typical full color OLED devices are constructed of pixels having three subpixels that are red, green, and blue in color. Such an arrangement is known as an RGB design. An example of an RGB design is disclosed in U.S. Pat. No. 6,281,634. Full color organic electroluminescent (EL) devices, have also recently been described that are constructed of pixels having four subpixels that are red, green, blue, and white in color. Such an arrangement is known as an RGBW design. An example of an RGBW device is disclosed in commonly-assigned U.S. Patent Publication 2002/0186214A1. In an RGBW device, a high efficiency white emitting pixel is used to display a portion of the digital image content. This results in improved power consumption relative to an RGB constructed of similar OLED materials. However, the red, green, and blue subpixels do not improved in efficiency in this design. Therefore, no power savings are achieved for displaying any portion of the digital image content that is pure red, pure blue, or pure green in color such as, for example, icons and toolbars commonly used in many personal digital assistant (PDA), cell phone, or digital camera applications. Furthermore, the addition of the fourth subpixel results in all red, green, and blue subpixels having to be made small to fit the same total number of subpixels per unit area and achieve the same device pixel resolution as a comparable RGB device. This results in the current density per unit area of the associated red, green, and blue subpixels having to be increased in order to display the same brightness for this pure red, pure blue, or pure green content. It is known that OLED devices degrade, or become less efficient, more quickly as current density is increased. For an RGBW display, this may result in content which is pure red, pure green, or pure blue, and appearing frequently such as, for example, icons and toolbars causing image burn-in quicker than in an equivalent RGB display and therefore overall device lifetime is reduced.
Therefore, an RGBW device with improved efficiency and lifetime for the red, green, and blue subpixels is desired.